28
ObservAir
Operating Manual
We recognize that regulatory monitoring stations with open data
access may not be available in many areas. Span calibration of the
ObservAir is a highly involved process that requires specialized equipment.
Some basic validation procedures may be added to this manual in the
future, but for the moment, please contact
to inquire about
our calibration services.
4.3.
Flow rate calibration
The flow rate sensor is calibrated at DST prior to shipment, but sensor
output may drift over long periods (weeks or months of operation) and
should be re-calibrated periodically using the instructions below:
1.
Turn on the ObservAir and allow it to warm-up for at least 15 minutes.
2.
Connect ObservAir to a serial monitor (Section 3.6).
3.
Set the ObservAir to the desired flow rate setting using the appropriate
serial command.
4.
Connect a primary flow calibrator, such as a Gilian Gilibrator, to the
ObservAir’s inlet such that the sensor’s intake flow is measured.
5.
Collect at least 5 flow rate measurements using the calibrator. The flow
rate measurements should not vary by more than ±1 ccm. Take the
average of all calibrator measurements: this is the reference value
(
FR
ref
).
6.
From the serial output, collect at least 5 flow rate measurements from
the ObservAir and take the average: this is the measured value (
FR
m
).
7.
Calculate the calibration factor:
FR
cal
=
FR
ref
/FR
m
8.
Update the sensor’s FRcal factor using the serial command (Section
3.6.3).
9.
Repeat Steps 5 and 6 until the reference and measured values agree
within 2 ccm.
If a primary flow calibrator is not available, another flow rate sensor may be
used, but clearly the calibration is only as good as the reference
measurements collected.
4.4.
Leak check
The ObservAir incorporates a monolithic design architecture that is more
robust than traditional instruments and less susceptible to leakage.
However, it is best practice to periodically check the integrity of the closed
flow path using the instructions below:
29
ObservAir
®
Operating Manual
1.
Turn off the ObservAir
2.
Replace the filter
3.
Plug the ObservAir’s exhaust port. A simple plug may be fashioned by
tying a knot in a length of sample line (soft rubber tubing).
4.
Connect a vacuum gun (e.g. a MityVac) to the inlet nozzle using a length
of sample line. The vacuum gun must be outfitted with a pressure
gauge.
5.
Pump the gun until you achieve a vacuum pressure of –10 inch of H
2
O.
Note that the pressure may ‘bounce back’ after every pump – this is
normal and is a result of the filter’s flow resistance.
6.
Monitor the vacuum pressure for about 1 minute to ensure that it
remains constant (the sensor is holding vacuum, and therefore not
leaking). The vacuum pressure may drop by up to –1 inch of H
2
O over
60 seconds. If the sensor meets or exceeds this criterium, it has passed
the leak check and may be deployed.
7.
If the sensor fails the leak check, first tighten the thumbscrew by 1 full
turn and repeat steps 5 and 6.
8.
If the sensor still fails, replace the filter and repeat steps 5 to 7.
9.
If the sensor fails after these attempts, contact DST Technical support
at
(please include the unit(s) serial numbers in the
subject line).
Note:
The ObservAir’s micropump can achieve a static vacuum pressure of
~2 inch of H
2
O and operates at << 1 inch of H
2
O when monitoring air
pollution. Therefore, some small vacuum loss is allowed at the –10 inch of
H
2
O setpoint, as this will not result in any perceptible leakage during normal
operation.